Young star clusters give birth to rogue planetary-mass objects

(Press-News.org) Planetary-mass objects (PMOs) are cosmic nomads: they drift freely through space, unbound to any star, and weigh less than 13 times the mass of Jupiter. While they have been spotted in abundance in young star clusters such as the Trapezium Cluster in Orion (Fig. 1), their origin has puzzled scientists. Traditional theories have suggested that they might be failed stars or planets ejected from their solar systems.

An international team of astronomers, in collaboration with the University of Zurich (UZH), has used advanced simulations to demonstrate that these enigmatic objects can form directly from the violent interactions of disks around young stars. “PMOs don’t fit neatly into existing categories of stars or planets,” said Lucio Meyer from the UZH, corresponding author of the study. “Our simulations show they are probably formed by a completely different process.”

How disks collide to create PMOs

Using high-resolution hydrodynamic simulations, the team from the University of Zurich, the University of Hong Kong, the Shanghai Astronomical Observatory, and the University of California Santa Cruz, recreated close encounters between two circumstellar disks – rotating rings of gas and dust that surround young stars. When these disks pass close to each other, their gravitational interactions stretch and compress the gas into elongated “tidal bridges.”

The simulations revealed that these bridges collapse into dense filaments, which further fragment into compact cores. When the mass of the filaments exceeds a critical threshold for stability, they produce PMOs with masses of about 10 Jupiters. Up to 14% of PMOs form in pairs or triples, which explais the high rate of PMO binaries in some clusters. Frequent disk encounters in dense environments such as the Trapezium Cluster could generate hundreds of PMOs.

Why PMOs are unique

PMOs form along stars, inheriting material from the outer edges of circumstellar disks. PMOs move in synchrony with the stars in their host cluster, unlike ejected planets. Many PMOs retain gas disks, suggesting the potential for moon or even planet formation around these nomads.

“This discovery partly reshapes how we view cosmic diversity,” said co-author Lucio Mayer. “PMOs may represent a third class of objects, born not from the raw material of star forming clouds or via planet-building processes, but rather from the gravitational chaos of disk collisions.”

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